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A research participant demonstrates the motion used to assess lumbopelvic control during a stepping movement, in this case mimicking the start of a pitch.

COLUMBUS, Ohio – Scientists who sequenced the genome of the Antarctic midge
suspect the genome’s small size – the smallest in insects described to date –
can probably be explained by the midge’s adaptation to its extreme living
environment.

The midge is a small, wingless fly that spends most of its
two-year larval stage frozen in the Antarctic ice. Upon adulthood, the insects
spend seven to 10 days mating and laying eggs, and then they die.

Its genome contains only 99 million base pairs of
nucleotides, making it smaller than other tiny reported genomes for the body
louse (105 million base pairs) and the winged parasite Strepsiptera (108
million base pairs), as well as the genomes of three other members of the midge
family.

David Denlinger

The midge genome lacks many of the segments of DNA and other
repeat elements that don’t make proteins, which are found in most animal
genomes. The lack of such “baggage” in the genome could be an evolutionary
answer to surviving the cold, dry conditions of Antarctica, said senior author David Denlinger,
Distinguished Professor of entomology
and of evolution, ecology and organismal biology
at The Ohio State University.

“It has really taken the genome down to the bare bones and
stripped it to a smaller size than was previously thought possible,” Denlinger
said. “It will be interesting to know if other extremophiles – ticks,
mites and other organisms that live in Antarctica – also have really small
genomes, or if this is unique to the midge. We don’t know that yet.”

Once called “junk DNA,” these DNA segments and repeat
elements in genomes are now known to have important functions related to gene
regulation. They also are implicated in many disease processes. So could a
bare-bones genome be the secret to midge survival?

“We don’t yet understand what the implications are of not
having all that extra baggage. It seems like a good thing in many ways, but
organisms do get some beneficial things from this baggage, too,” Denlinger
said.

The midge genome is small in architecture but not in the
number of genes, the researchers noted: The Antarctic midge genome, like
genomes of other flies, contains about 13,500 functional genes.

Denlinger has studied the Antarctic midge for many years,
zeroing in on the insect’s unusual stress responses, including the activation
of heat-shock
proteins. Most animals turn on these proteins only when they’re under acute
stress – particularly when they’re exposed to extremely high or low
temperatures – and quickly turn them off when the stress has passed. But
heat-shock proteins are activated constantly during the Antarctic midge’s larval
stage – a trait scientists believe is linked to its survival in harsh
conditions.

Denlinger’s lab has cloned and studied several genes
connected to these proteins. “But sequencing the genome gives us access to a
broader suite of many other closely related genes that we didn’t have access to
before,” he said.

The research also reveals a host of genes called aquaporins, which are
involved in water transport into and out of cells. These genes and the proteins
they make are also players in the midge’s survival in Antarctica. Most insects
can survive losing about 20 percent of the water in their bodies’ cells, but
these midges tolerate a loss of up to 70 percent of their water.

“They look like dried up little raisins, and when we pour
water on them they plump up and go on their merry way,” Denlinger said. “Being
able to survive that extreme level of dehydration is one of the keys to
surviving low temperatures. This midge has some mechanism that enables it to
both be dehydrated and stay alive, with its cells functioning normally.”

In the Antarctic ecosystem, these midges eat bacteria and
algae as well as nitrogen-rich waste produced by penguins. No other species
preys on them, and Denlinger’s lab has not identified any pathogens that might
endanger their lives.

But they do have this incredible ability to survive a
significant threat: the dry, deep freeze and high levels of ultraviolet
radiation of Antarctica, and precisely how they do that remains, at least in
part, a mystery. Though the survival question alone drives Denlinger’s work,
the research could have implications for humans in the long term by revealing
how human tissue harvested for transplant could be sustained in cold storage.

“How does the midge regulate its physiology so it can
survive in those kinds of low temperature extremes?” he said. “Having
heat-shock proteins turned on all the time could offer some clues about how you
might be able to preserve other tissues for a long time. Midges have figured
out how to do that, so that means it is possible for some animal tissues to survive
freezing temperatures.”